1. Field of the Invention
The invention relates to a substrate material for X-ray optical components, comprising a glass ceramic material with a thermal expansion |α|<5×10−6 K−1 in a predetermined temperature range of, a method for producing such a substrate material, and a use of such a substrate material.
2. Description of the Related Art
X-ray optical components are especially of particular interest in the field of X-ray lithography. This applies in particular to lithography with soft x-rays, i.e., EUV lithography in the wavelength region of 10 to 30 nm. Mirrors with the highest possible reflectivity in the X-ray region are used as optical components in the field of X-rays. Such X-ray mirrors can be operated close to perpendicular incidence or in grazing incidence, namely as so-called normal or grazing incidence mirrors.
X-ray mirrors comprise a substrate and, based thereon, a multilayer system. The multilayers are also known as “Distributed Bragg Reflectors” (DBRs). The multilayers allow the realization of mirrors with high reflectivity in the X-ray region in the case of non-grazing incidence, i.e. in the normal incidence operation. A grazing incidence mirror has a simpler structure than a normal incidence mirror because the grazing incidence mirror has fewer of the multilayers.
X-ray mirrors that are operated close to perpendicular incidence (normal incidence) are preferred over mirrors that are operated with grazing incidence in cases where high imaging quality by low aberrations is required, e.g., in imaging systems such as projection lens systems for EUV lithography systems.
In order to increase the reflectivity of grazing incidence mirrors, the substrates of these mirrors can also be provided with a multilayer system.
Reference is hereby made to DE 199 23 609 A1 and U.S. application Ser. No. 09/322,813, as filed with the US Patent Office on 28 May 1999 under the title “Reduction objective for extreme ultraviolet lithography”, now U.S. Pat. No. 6,244,717, concerning projection lens systems for EUV lithography and related X-ray optical components, the scope of disclosure of which is hereby fully incorporated into the present application.
Multilayer systems based on the substrate can be layer systems comprising layer pairs of Mo/Si, Mo/Be, or MoRu/Be, and having 40 to 100 such layer pairs. Such systems provide reflectivity in the region of 70 to 80% in the EUV range λR=10 to 30 nm. Depending on the wavelength of the light to be reflected, layer systems of other materials can be used.
The high reflectivity of the layer stack is achieved by phase-adjusted superposition and constructive interference of the partial wave fronts reflected on the individual layers. The layer thicknesses must be controlled to be within about 0.1 nm of a desired thickness.
The necessary preconditions for the achievement of high reflectivity are sufficiently low layer and substrate roughness in the high spatial frequency roughness (HSFR) range. Depending on the approach, this spatial frequency range leads to loss of light by scattering outside of the image field of the lens system or by disturbance of the microscopically phase-correct superposition of the partial wave trains. The relevant spatial frequency range is downwardly limited by the criterion of scattering outside of the image field and depending on the application it is typically at EUV wavelengths in the region of some μm. Generally, no limit is specified towards high spatial frequencies. A useful limit value lies in the range of half the wavelength of the incident light, because higher spatial frequencies are no longer seen by the incident photons. HSFR is usually measured with atomic force microscopes (AFM) which have the required lateral resolution.
Concerning the definition of HSFR, MSFR and fine surface figure error, as used herein, reference is hereby made to:    U. Dinger, F. Eisert, H. Lasser, M. Mayer, A. Seifert, G. Seitz, S. Stacklies, F. J. Stiegel, M. Weiser, “Mirror Substrates for EUV-lithography; progress in metrology and optical fabrication technology”, Proc. SPIE Vol. 4146, 2000, the scope of disclosure of which is hereby fully incorporated into the present application.
The fine surface figure error range according to the above publication reaches from the optically free diameter, i.e. the aperture of the mirror, up to 1 mm of roughness wavelength. MSFR comprises the roughness wavelengths from 1 μm to 1 mm. The HSFR range comprises roughness wavelengths of 10 nm to 1 μm.
Other X-ray optical components may require a structure that is characterized by high reflectivity and low thermal expansion. Examples are a reticle mask for an EUV projection illumination system, a mirror with raster elements, a so-called optical integrator or a collector mirror of an EUV illumination system. Reference is hereby made to DE 199 03 807 A1 and U.S. application Ser. No. 09/305,017, as filed with the US Patent Office on 4 May 1999 under the title “Illumination system particularly for EUV lithography”, now U.S. Pat. No. 6,198,793, the scope of disclosure of which is hereby fully incorporated into the present application.
Substrate materials for multilayer systems which are based on such materials are currently crystalline silicon, amorphous and semi-crystalline glasses such as the glass ceramic material ZERODUR® of Schott-Glas, Mainz.
In the field of high spatial frequency roughness (HSFR), a sufficient value of 0.1 nm rms for example can be achieved with classical superpolishing methods both on silicon as well as ZERODUR® and amorphous glasses. Since these methods lead at least in the aspheric regions generally to a deterioration of the fine surface figure error, i.e. defects in the low spatial frequency region, and in the mid spatial frequency roughness (MSFR) range to a deterioration of the long-wave shares in MSFR, it is usually necessary to provide a roughness-maintaining fine correction process after the superpolishing process.
Surface figure error and also the long-wave shares in MSFR (mm waves) can be brought to specification with beam processing methods, i.e. the IBF (ion beam figuring). The advantage of this method is that tools used in the method can sit closely to the working surface, and so the tools can fit snugly on the typically aspheric surfaces. These beam processing methods are based on sputtering processing. The global and local sputtering rates depend on the physical and chemical bonding conditions in the solid body to be processed.
Whereas in single-crystalline silicon the additional energy introduced by the incident ions leads to a surface reorientation with the result of improved roughness, a slight deterioration of HSFR is observed in amorphous glass from approx. 0.06 to 0.15 nm rms. In semi-crystalline glass ceramic material such as ZERODUR® for example, with a crystalline size of greater than 50 nm, there was a serious deterioration from 0.1 to 0.4 nm rms.
Glass ceramic materials with a crystallite size of high quartz mixed crystals ≧80 nm and a mean coefficient of thermal expansion α20° C.-700° C.<0.5·10−6/K are known from DE 199 07 038 A1.
Heat-resistant ceramic materials with a mean surface roughness ≦0.03 μm are shown by JP-A-04-367538. JP-A-04-367538 does not provide any disclosure concerning the mean thermal expansion. Furthermore, JP-A-04-367538 makes no statements as to the spatial frequency range in which these roughness values are achieved.
Although the single-crystalline silicon is a suitable carrier under the aspect of the roughness requirements that are demanded for the substrate material, single-crystalline silicon has a mechanical anisotropy however and only allows for small mirror sizes due to its property as single crystal. Although the disadvantage of a coefficient of thermal expansion α which is higher than that of glasses can be compensated partly by a considerably higher thermal conductivity and suitable cooling, it still requires a high amount of technical effort. Silicon as a substrate is currently only considered in the case of very high thermal loads such as in illumination systems.
Although the thermal expansion and the roughness in the HSFR range are unproblematic when using amorphous glasses with low thermal expansion such as glasses as described in U.S. Pat. No. 2,326,059, sufficient surface figure error and MSFR values cannot be reached because the lamellae-like striated structure or Schlieren-structure of amorphous glass with negligibly low thermal expansion has a disadvantageous effect in these frequency ranges. As a result, these layers of a thickness of approx. 0.1 mm on moderately curved mirror surfaces lead to non-correctable surface modulations in the mm range with amplitudes of a number of nanometers far outside the values required for EUV-lithography. This effect is also observed in ion-beam-based production processes.
Although the semi-crystalline glass ceramic material ZERODUR® with crystallite sizes of greater than 50 nm has the desired low coefficient of thermal expansion, it leads to excessive roughness values in the HSFR range in the final beam processing process.
It is the object of the present invention to provide a substrate material for X-ray optical components which has a low coefficient of thermal expansion like glass for example, but on the other hand ensures a sufficient surface quality of the X-ray optical components after the necessary surface processing steps.